The present invention relates to a developing unit for a recording apparatus such as a copying machine and and electrophotographic printer.
An electrophotographic recording apparatus generally includes, as its principal components: a photosensitive drum having an image forming member made of, for example, a photosensitive layer of amorphous selenium formed on a metallic cylindrical surface; an electrification unit for uniformly electrifying the surface of the photosensitive drum by corona discharge, for example in order, to photosensitize the photosensitive drum; an exposing unit to form an electrostatic latent image on the surface of the photosensitive drum, thus photosensitized by irradiating a laser beam modulated by information to be recorded; a developing unit for developing the electrostatic latent image, which has been formed by the exposing unit using toner; a transferring unit for transferring the toner image formed by the developing unit to paper; and a fixing unit for fixing the toner image, which has been transferred to the paper by the transferring unit through for example, a heat melting process.
In the developing unit having the above arrangement, the toner having an electric charge of an opposite polarity to that of the electrostatic latent image formed on the photosensitive drum is adsorbed electrostatically to the latent image. Generally, a powder toner is used which is mixed with for example an iron powder, called the "carrier", to provide the powder toner with the electric charge. This mixture is known as a "two-component developer" or simply a "developer". When the toner and carrier are mixed in the developing unit, frictional electrification occurs which is used to provide the electric charge for the developer.
Generally, a mixing ratio of the toner (toner concentration) is expressed by weight percentage, and is, for example, 4%. Since the toner concentration is gradually decreased during use, the toner concentration must be observed and toner supplied when the toner concentration reaches a predetermined limit.
In an electrophotographic recording apparatus which allows the use of several kinds of paper having different widths, the toner concentration in the developing unit becomes non-uniform in a paper width direction if paper having a width narrower than the maximum recordable width are used for a long time, and subsequently if a paper having a width wider than the width of the narrower paper is used, a difference in the recording density will occur in a paper width direction of the wider paper.
Therefore, particularly in a recording apparatus which uses papers of different widths, a means must be provided to solve the non-uniformity of toner concentration in the paper width direction.